What the exam asks
- Read and rotate trees: Identify the most closely related species by locating their most recent common ancestor (node), recognizing that rotating branches around a node does not change relationships.
- Build or complete a model: Place species or shared derived characters onto a blank template in your paper FRQ booklet using a character matrix or a sequence divergence table.
- Evaluate evidence: Explain why molecular data (DNA, RNA, or amino acid sequences) provide more reliable phylogenetic evidence than morphology, and explain how trees are revised when new evidence emerges.
Core ideas
Trees vs. cladograms as testable hypotheses
Both phylogenetic trees and cladograms are branching diagrams that represent testable, revisable hypotheses about evolutionary relationships among lineages. Because they are hypotheses, scientists continually update them as new fossils are discovered or new genomes are sequenced.
| Feature | Cladogram | Phylogenetic Tree |
|---|---|---|
| Branching pattern | Shows relative order of divergence | Shows relative order of divergence |
| Branch lengths | Arbitrary; do not represent time or amount of change | Scaled to represent evolutionary time (calibrated by fossils) or amount of genetic change (via a molecular clock) |
Reading nodes, clades, and out-groups
- Nodes (branch points): Represent a speciation event and the most recent common ancestor (MRCA) of all descendant lineages branching from that point. Two taxa are more closely related to each other than to a third taxon if they share a more recent node.
- Clade (monophyletic group): Any ancestor and all of its descendants. You can snip a clade off a tree with a single cut.
- Out-group: A lineage that is least closely related to the remainder of the organisms (the in-group) in the analysis. It diverges earliest (closest to the root) and serves as a reference point to distinguish ancestral traits from derived traits.